lesson of our ignorance has not been learned by biologists. We encounter, not infrequently, the assertion that life is nothing but a series of physical phenomena; or, on the other hand, what is less fashionable science just now, that life is due to a special vital force. Such assertions are thoroughly unscientific; most of them are entirely, the remainder nearly worthless. Of what seems to me the prerequisites to be fulfilled before a general theory of life is advanced, I have written elsewhere.5

II. CONCEPTION OF DEATH.

My thesis reads: There are two forms of death. These are first, the death of the single cells; second, the death of multicellular organisms. Death in the one case is not homologous with death in the other.

Weismann assumed the complete homology of the two forms of death. Without this assumption, his hypothesis of the immortality of unicellular organisms falls to the ground and with it falls the entire superstructure of his speculations upon germ plasm. Oscar Hertwig (Zeit und Streitfragen, Heft 1) has already expounded, very clearly, the dependence of the theory of germ plasm upon the hypothesis of unicellular immortality; it would, therefore, be superfluous to discuss it here.

The conception of the biological problem of death, to which I still hold, was formed several years before Weismann's first publication, which appeared in 1882, with the title, " Ueber die Dauer des Lebens." He has further defended his view in his article, "Ueber Leben und Tod" (1884), and has steadfastly adhered to it since. In the years 1877-1879 I published my theoretical interpretation of the problem. This interpretation became the starting point of elaborate special investigations, by which I endeavored to advance the solution of the problem and, in fact, observation and experiment have confirmed the 5 C. S. Minot, On the conditions to be filled by a theory of life, Proc. Amer. Assoc. Adv. Sc., XXVIII, 411.

Proc. Boston Soc. Nat. Hist., XIX, 167; XX, 190.

original thesis. Moreover, in an especial short article I have directed attention to the fact that Weismann has not consid ered the essential issue of the problem. The difficulties pointed out still remain, and, according to my conviction, cannot be removed. Weismann passes these difficulties by and carries out his speculations without first securing a basis for them. His method is illustrated by the following quotation : "I have, perhaps, not to regret that I cannot here discuss the article referred to (Minot's Article in Science, Vol. IV, p. 398); nevertheless, almost all objections which are there made to my views are answered in the present paper." (Weismann, Zur Frage nach der Unsterblichkeit der Einzelligen, Biol. Centralbl., IV, 690, Nachschrift). I have studied the paper with conscientious care and cannot admit that the objections have been answered. On the contrary, I maintain now, as formerly, the judgment: "He misses the real problem." For this reason I hold it to be unnecessary to discuss the details of Weisman's exposition, because—if I am right—he has not considered the actual problem of death at all. "He misses the real problem." The following reasoning leads to this decision: Protozoa and Metazoa consist of successive generations of cells; in the former the cells separate; in the latter they remain united; the death of a Protozoa is the annihilation of a cell, but the death of a Metazoon is the dissolution of the union of cells. Such a dissolution is the result of time, that is to say, of the period necessary to the natural duration of life, and we call it, therefore, "natural death." Moreover, we know that natural death is brought about by gradual changes in the cells until, at last, certain cells, which are essential to the preservation of the whole, cease their functions. Death, therefore, is a consequence of changes which progress slowly through successive generations of cells. These changes cause senescence, the end of which is given by death. If we wish to know whether death, in the sense of natural death, properly so called, occurs in Protozoa or not, we must first pos

7

Journal of Physiology, XII, and Proc. A. A. A. S., XXXIX, (1890).

sess some mark or sign, by which we can determine the occurrence or absence of senescence in unicellular organisms.

If we ap

Around this point the whole discussion revolves. Certainly a simpler and more certain conclusion could hardly be drawn than that the death of a Metazoon is not identical, i. e., homologous with the death of a single cell. Weismann tacitly assumed precisely this homology, and bases his whole argument on it. In all his writings upon this subject, he regards the death of a Protozoon as immediately comparable with the death of a Metazoon. If we seek from Weismann for the foundation of this view we shall have only our labor for our pains. Starting from this view Weismann comes to the strictly logical conclusion that the Protozoa are immortal. This is a paradox! In fact, if one compares death in the two cases, from Weismann's standpoint, then we must assume a difference in the causes of death, and conclude that the cause in the case of the Protozoa is external only, while in the Metazoa it is internal only, for, of course, we may leave out of account the accidental deaths of Metazoa. proach the problem from this side, we encounter the following principal question: Does death from inner causes occur in Protozoa? Weismann gives a negative answer to this question, with his assertion that unicellular organisms are immortal. The assertion remains, but the proof of the assertion is lacking. In order to justify the assertion, it must be demonstrated that there does not occur in Protozoa a true senescence, showing itself gradually through successive generations of cells. Has Weismann furnished this demonstration? tainly not. He has, strictly speaking, not discussed the subject. It is clear that we must first determine whether natural death from senesence occurs in Protozoa or not, before we can pass to a scientific discussion of the asserted immortality of unicellular beings. The problem cannot be otherwise apprehended. Weismann has not thus conceived it, therefore the judgment stands against him: he misses the real problem.

Cer

Senesence has been hitherto little investigated; for many years I have been studying it experimentally and have tried

to determine its exact course. My paper, "Senesence and Rejuvenation," affords evidence of new facts proven by these experiments. I believe I have thus won the right to oppose my view to the pure speculations of Weismann.

(To be continued.)

LOST CHARACTERISTICS.

BY ALPHEUS HYATT.

Dr. Minot having noticed, in the translation of his article "On Heredity and Rejuvenation," an accidental omission of quotation of work done by paleontologists on the loss of characteristics in the development of animals, has most courteously asked me to follow his essay by an article dealing with this question. I gladly avail myself of this opportunity on account of the advantages offered where similar subjects can be consecutively treated from different points of view, and because Dr. Minot's article, on account of his great and deserved reputation in embryology, will reach the students of existing biological phenomena, and perhaps induce some of them to read a connected publication.

The loss of characteristics is not so readily observed by a student of the biology of existing animals or neobiologist, as by the paleobiologist or student of fossils, because the latter necessarily deals with series of forms often persisting through long periods of time, and is led, especially if he follow more recent methods of research, to study these in great detail. The observer of these remains is not, as is falsely imagined, limited to fragments, but can and does work out of the hard matrix the external skeletons or shells even of embryos, and can, in the corals, brachiopoda, mollusca, echinodermata and even in protozoa, follow the entire life history of these parts in the individual. He has also the further advantage of availing himself of the knowledge amassed by the neobiologist and necembryologist, the works of Cope, Beecher, Schuchert, Gurley,

Jackson and others, written in the last thirty years in this country and in Europe. The new school of Paleobiology also insists upon the close study of series of forms and rejects the methods usually pursued by the neoembryologist, who, as a rule, selects his objects of study and pursues his comparisons upon the old basis of comparative anatomy and with but little regard to the serial connections of forms. The importance of studying the seriality in structure of the members of the same group, those gradations, which lead from one variety to another, one species to another, one genus to another, until they may end in highly differentiated and often degraded offshoots, with as strange and unique developments as they have adult characters, seems not, as yet, to have attracted the attention of the students of development among recent animals as it has that of paleobiologists. The prevalent modes of study of living types has consequently led to noticing the phenomena of omission of hereditary characters only in an isolated way, and from the time of Balfour's "Comparative Embryology" these omissions occurring in the embryo have been named abbreviations, shortenings and omissions of development, and various attempts have been made to explain them upon more or less general grounds of inference. Prof. Cope and the writer and some other authors have been for a number of years publishing observations upon this class of phenomena under the title of the law of acceleration, asserting that in following out the history of series in time, or of existing series in structure, there was observable a constant tendency in the successive members (species, genera, etc.) of the same natural group to inherit the characters of their ancestors at earlier stages than those in which they appeared in these ancestors. That as a corollary of this tendency, the terminal forms eventually skipped or omitted certain ancestral characteristics, which were present in the young of the preceding or normal forms of the same series. and also in the adult stages of development of more remote ancestors of the same genetic stock or series. This law has since been independently rediscovered by several other naturalists, notably Würtemburger in Germany, and Buckman in England. The writer has lately christened this as the law of